1. Field of the Invention
The present invention relates to a direct electrostatic printing method, in which a stream of computer generated signals, defining image information, are converted to a pattern of electrostatic fields to selectively control the deposition of charged toner particles in an image configuration directly onto an information carrier.
2. Description of the Related Art
Of the various electrostatic printing techniques, the most familiar and widely utilized is xerography, wherein latent electrostatic images formed on a charge retentive surface, such as a roller, are developed by a toner material to render the images visible, the images being subsequently transferred to plain paper. This process is called an indirect process since the visible image is first formed on an intermediate photoreceptor and then transferred to a paper surface.
Another method of electrostatic printing is one that has come to be known as direct electrostatic printing, DEP. This method differs from the aforementioned xerographic method in that charged toner particles are deposited directly onto an information carrier to form a visible image. In general, this method includes the use of electrostatic fields controlled by addressable electrodes for allowing passage of toner particles through selected apertures in a printhead structure. A separate electrostatic field is provided to attract the toner particles to an image receiving substrate in an image configuration.
The novel feature of direct electrostatic printing is its simplicity of simultaneous field imaging and toner transport to produce a visible image on the substrate directly from computer generated signals, without the need for those signals to be intermediately converted to another form of energy such as light energy, as is required in electrophotographic printers, e.g., laser printers.
U.S. Pat. No. 5,036,341 granted to Larson, discloses a direct printing method which begins with a stream of electronic signals defining the image information. A uniform electric field is created between a high potential on a back electrode and a low potential on a toner carrier. That uniform field is modified by potentials on selectable wires in a two dimensional wire mesh array placed in the print zone. The wire mesh array consists of parallel control wires, each of which is connected to an individual voltage source, across the width of the information carrier. A drawback of such a device is that, during operation of the wire mesh array, the individual wires can be sensitive to the potentials applied on adjacent wires, resulting in undesired printing due to interaction or cross-talk between neighboring wires.
U.S. Pat. No. 5,121,144, also granted to Larson, discloses a control electrode array formed of a thin sheet-like element comprising a plurality of addressable control electrodes and corresponding voltage sources connected thereto. The control electrode array may be constructed of a flexible, electrically insulating material and overlaid with a printed circuit such that apertures in the material are arranged in rows and columns and are surrounded by electrodes. An electrostatic field on the back of electrode attracts toner particles from the surface of the particle carrier to create a particle stream toward the back electrode. The particle stream is modulated by voltage sources which apply an electric potential to selected control electrodes to produce electrostatic fields which permit or restrict transport of toner particles from the particle carrier through the corresponding apertures. The modulated streams of charged particles allowed to pass through the selected apertures impinge upon an information carrier interposed in the particle stream to provide line-by-line scan printing to thereby form a visible image.
The control electrodes are aligned in several transverse rows extending perpendicularly to the motion of the information carrier. All control electrodes are initially at a white potential V.sub.w to prevent all particle transport from the particle carrier. As image locations on the information carrier pass beneath apertures, corresponding control electrodes are set to a black potential V.sub.b to produce an electrostatic field which draws the toner particles from the toner carrier. Charged toner particles allowed to pass through the apertures are subsequently deposited on the information carrier in the configuration of the desired image pattern. The toner particle image is then made permanent by using heat and pressure to fuse the toner particles on the surface of the information carrier.
Common to all electrostatic printing methods is that toner particles are transported along a substantially straight trajectory coinciding with a central axis of the aperture, and impinge upon the information carrier at a substantially right angle, resulting in that the addressable area of each aperture is limited to a single "dot," having a predetermined, nonvariable extension on the information carrier. The number of dots which can be printed per length unit in a longitudinal direction, i.e., parallel to the motion of the information carrier, can be increased by lowering the speed of the information carrier through the print zone, thereby allowing a larger number of print sequences per length unit to be performed.
A drawback of the aforementioned method is that the number of dots which can be printed per length unit in a transverse direction, i.e., perpendicular to the motion of the information carrier, is strictly limited by the number of apertures that can be arranged in the control array.
Hitherto, the transverse print addressability has generally been improved by increasing the number of apertures and related control electrodes across the control array, resulting in higher manufacturing cost and more complicated control function. However, increasing the number of apertures results in the apertures having to be spaced closer to each other, thereby causing the control electrodes to not only act on their associated aperture but also to substantially influence all adjacent apertures, due to the interaction between adjacent electrostatic fields. This results in a degradation of the print quality and readability.
Further, to increase transverse print resolution, i.e., the number of distinguishable dots that can be printed per length unit in a transverse direction across the information carrier, it is also essential to provide dots that are sufficiently small to be deposited adjacent to each other without overlapping by than half a dot width. For instance, to obtain a print resolution of 600 dots per inch (DPI), the overlap width of two adjacent dots might not exceed 1/600 inch, i.e., about 42 microns, and the size of a dot might be in the order of 60 to 80 microns to be discernible on the image configuration.
Hitherto, dot size has been decreased by reducing the amplitude or the pulse width of the electrostatic field controlling the corresponding aperture in order to reduce the amount of toner particles passing through the aperture. However, this may not only influence the size of the dots, but may even considerably affect their density and uniformity.
Therefore, regardless of the design of the control electrode array, the present applicant has perceived a need to improve the print resolution of direct printing methods by enhancing transverse print addressability while reducing the dot size, without increasing the number of apertures required.